Liquid crystal displays (LCDs) known from prior art usually contain an assortment of different functional layers, like for example alignment layers, colour filters and retardation layers. Retardation films are used for example to convert between linear and circular polarised light and to compensate the deteriorating contrast and colour of the LCD at wide viewing angles. However, the optical dispersion of conventional retardation films as used in prior art often results in non-perfect conversion, i.e. not all frequencies of visible light are converted between linear and circular polarised states. This is especially disadvantageous for colour LCDs of the active matrix type, where a colour filter is applied so that different pixels of the display in the adressed state show different primary colours (R, G, B).
One aim of the present invention is to provide a retardation film for use in LCDs, especially in active matrix colour LCDs, which does not have the drawbacks of prior art films mentioned above, and in particular allows more efficient conversion between linear and circular polarised states for light of different wavelengths. Another aim is to provide advantageous methods and materials for the preparation of such a retardation film. Another aim is to provide advantageous uses of such a retardation film. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors have found that these aims can be achieved by providing a patterned or pixelated retardation layer prepared by the methods as described below, which comprises a pattern of different regions or pixels with different retardation. The pixelated retardation layer can be applied to an LCD comprising a colour filter such that each pixel of the retardation layer has the correct retardation for the light passing through the corresponding pixel of the colour filter. In this way the polarised light passing through the display can be more efficiently converted. The pixelated retarder according to the present invention is preferably prepared by exposing a polymerizable liquid crystal (LC) material incorporating compounds with photosensitive groups to UV light.
It was also found that the methods and materials according to the present invention allow the preparation of retardation layers with different orientation of the LC material, for example planar and splayed layers. Thereby patterned films with a pattern of regions with different orientation and/or different retardation can be prepared. It is also possible to prepare a polymerised LC layer that acts as alignment layers for a subsequent layer, and stacks of films or layers having different orientation and/or retardation.
EP 02019792.7 discloses an optical retardation film with a retardation pattern comprising polymerised nematic LC material, and methods for its preparation by varying the polymerisation conditions and/or the composition of the polymerisable LC material. However, it does not disclose the films or methods according to the present invention.